The inventor and his associates at A.E.A. have long been developing engine designs for ultra-lean combustion. Much of the work is described as "High Swirl Very Low Pollution Piston Engine Employing Optimizable Vorticity" U.S. Pat. No. 4,344,394. The technology described in this patent permits engines to be operated at exceptionally lean air/fuel ratios with efficiency at or approaching that of small automotive diesels and with exceptionally low NO.sub.x emissions. In an engine such as that described in U.S. Pat. No. 4,344,394 the cylinder-to-cylinder, cycle-to-cycle, and microscale volume mixing statistics on air/fuel ratio are much tighter than in the prior art, and the statistical variation of mixture motion inside the cylinder is also tightened. With such an engine, it has been found that the optimal fuel economy air/fuel ratio, using the Schweitzer Procedure for determining the true best economy mixture, occurs at very lean ratios. When the engine is operated richer than this optimal point, the cyclic variation of flame speed and cyclic variation of peak pressure observed in the engine is very much less than that characteristic of prior art engines. It has been found imperically that the point of optimal fuel economy (which is nearly the point for minimum NO.sub.x emissions) correlates over the speed load phase space with the onset of significant statistical variation of flame speed and peak pressure. The statistical variations of flame speed and peak pressure are not greater than the variations commonly encountered with ordinary mixtures at stoichiometric or slightly lean ratios, but the statistical variations can be measured and used as engine control inputs. It is highly desirable that the engine operate (except at very high loads where rich mixtures are required to make torque) at air/fuel ratios which are lean enough for a specific range of statistical variation of flame speed (not misfire). If the air/fuel ratio is controlled to achieve this borderline roughness, fuel economy is nearly optimized and NO.sub.x is nearly minimized. The adjustment of air/fuel ratio for borderline roughness gives the proper engine adjustment for efficiency and NO.sub.x control regardless of fuel type (e.g. gasoline vs. methanol) or intake air density, and efficiently compensates for variations in flame stability with internal or external EGR as a function of speed and load. The roughness control can also compensate for engine and ambient temperature.
It is therefore a purpose of the present invention to produce a control which reads a measure of flame speed or peak pressure, and adjusts air/fuel ratio so that the statistical variation of flame speed (or a reliably correlated measure of flame speed) achieves a specified degree of statistical variation. If the statistical variation is smaller than a set value, the control shifts the air/fuel ratio towards the lean side where statistical variation of flame speed is increased. If statistical variation of flame speed, or a measure of it, is excessive the mixture is shifted richer.
It is impossible to get the dynamic response of a roughness combustion control to be rapid enough to respond to the rapid load changes required for driveability if the roughness sensor acts alone. However, the roughness sensitive control can input a relatively slowly moving correction function for a faster automatic control system. Specifically, the control can continuously update a variable which multiplies the air/fuel ratio selected by a more rapid autonomous fuel/air metering controller. The autonomous fuel/air metering system can be built to respond to variations in speed, load, etc., and can have very rapid response. Therefore the control system of the current invention uses the roughness control as a correction function which adjusts the calibration of an automatic programmed air/fuel metering system continuously.
The advantages of NO.sub.x control and improved efficiency available with enleanment occur because the leaner mixtures have lower peak flame temperatures and hence lower dissociation losses and lower NO formation kinetic rates. These same effects can be obtained with introduction of EGR for a richer air/fuel ratio rather than with enleanment of the air/fuel ratio itself. It is, therefore, another purpose of the present invention to produce a control which reads the measure of flame speed or peak pressure and adjusts EGR input so that the statistical variation of flame speed achieves a specified degree of statistical variation. Such an EGR control can achieve the same advantages as enleanment with a super-homogeneous engine operated with in-cylinder flow control. The EGR control can also be useful as a drivability control for more conventional engines, and can, therefore, be applied to EGR controls on current vehicles as well as to EGR introduction to better-mixed systems.
A number of measures of flame speed, and hence cyclic variation of flame speed, are available. One can read the peak acoustic pressure at each exhaust blowdown since this peak pressure correlates inversely with flame speed. Another convenient measure of flame speed is the ionization resistance at the spark gap on refiring the plug 20 or 30 degrees after the initial ignition. Whichever combustion measure is chosen, the control system functions by taking a running total of a flame speed measure and controlling air/fuel ratio to adjust the flame speed measure variance to a set value.
Applied to an engine such as the ultra-homogenous variable restriction flap engine of U.S. Pat. No. 4,344,394 a roughness sensor control superimposed on a rapid response fuel/air metering system or EGR introduction system serves to optimize NO.sub.x and other emissions, maximize fuel efficiency, and compensate for variations in fuel, altitude, temperature and other variables.